Refrigeration system with thermal conductive defrost

ABSTRACT

A refrigeration system including a compressor, a condenser, and an evaporator in heat exchange relationship with a fluid deliverable to a space. The evaporator is connected to the condenser via an inlet line and to the compressor via a suction line. A refrigeration circuit is defined by the compressor, condenser, inlet line, evaporator, and suction line. A refrigeration mode of the refrigeration circuit transfers heat from the fluid to a refrigerant in the evaporator and accumulates frost on the evaporator. The evaporator, a bypass line connected to the suction line and to the inlet line, a portion of the suction line, and a portion of the inlet line define a defrost circuit that circulates refrigerant in a defrost mode to transfer heat from the refrigerant to the frost on the evaporator. The refrigerant flows by gravity from the evaporator to one of the portion of the suction line and the portion of the inlet line in the defrost mode.

BACKGROUND

The present invention relates to a defrost system for an evaporator of arefrigeration system. More particularly, the invention relates to arefrigeration system that includes a refrigeration mode that generatesfrost on an evaporator and a defrost mode that removes frost from theevaporator.

Refrigeration systems are well known and widely used in supermarkets andwarehouses to refrigerate food product displayed in a product displayarea of a refrigerated merchandiser or display case. Conventionalrefrigeration systems include an evaporator, a compressor, and acondenser. The evaporator allows heat transfer between a refrigerant anda fluid passing over coils of the evaporator. The evaporator transfersheat from the fluid to the refrigerant so that the fluid cools theproduct display area. The refrigerant absorbs heat from the fluid in arefrigeration mode. In the refrigeration mode the compressormechanically compresses the evaporated refrigerant from the evaporatorand feeds the superheated refrigerant to the condenser, which cools therefrigerant. From the condenser, the cooled refrigerant is fed throughone or more expansion valves to reduce the temperature and pressure ofthe refrigerant, and then the refrigerant is directed through theevaporator.

Since most evaporators operate at evaporating refrigerant temperaturesthat are lower than the freezing point of water (i.e., 32 degreesFahrenheit), water vapor from the fluid freezes on the evaporator coilsand creates frost. The frost decreases the efficiency of the heattransfer between the evaporator and the fluid, which causes thetemperature of the refrigerated space to increase above a desired level.Maintaining the correct temperature of the refrigerated space isimportant to maintain the quality of the stored food products. To dothis, the evaporators must be defrosted regularly in order toreestablish efficiency.

During defrost, the refrigeration system no longer refrigerates the foodproducts. Some existing refrigeration systems defrost the evaporatorusing a heating element in communication with the fluid to heat thefluid. The heated fluid then warms the outside of the evaporator abovethe freezing point to melt the frost. The heated fluid further depositsthe accumulated moisture as frost on cold surfaces of the refrigeratedmerchandiser, causing buildup of frost on the merchandiser. This methodalso results in wasted heat because some of the heated fluid escapesinto the product display area, potentially spoiling the food product.

Other conventional refrigeration systems include valves that allowsuperheated vapor to flow from a discharge line of the compressor intothe evaporator to defrost the coils. However, the process increasesenergy costs necessitated by operation of the compressors that compressthe superheated vapor.

Still other conventional refrigeration systems include a reservoir fromwhich evaporated refrigerant is drawn. These systems are sometimescalled “hot” refrigerant gas defrost systems. Typically, hot refrigerantfrom a common discharge manifold or from an upper part of a refrigerantreceiver is fed backward through the refrigeration system in a reverseflow to the evaporator to defrost the evaporator. The hot refrigerantgas is liquefied during its passage through the evaporator and thelatent heat is used to melt the frost on the evaporator coils. Theduration of the defrost period is directly proportional to therefrigerant mass flow. The higher the mass flow, the shorter the defrostperiod will be. However, the refrigerant mass flow depends on thecondensing pressure of the refrigeration system. During the colderperiods of the year the refrigerant mass flow is reduced, which therebyreduces the refrigerant mass flow and creates an economicallyunacceptable system. Further, the liquid refrigerant obtained duringdefrost of the evaporator is returned to the liquid line of therefrigeration system. This has a disruptive effect on the quality of theliquid refrigerant fed to the evaporators in normal operation (e.g.,“flash gas”, higher liquid refrigerant temperature, etc.). These reverseflow systems are often complex, difficult to maintain, and substantiallyadd to the cost of the refrigeration system.

SUMMARY

In one embodiment, the invention provides a refrigeration system thatincludes a compressor, a condenser fluidly connected to the compressor,and an evaporator. The evaporator is fluidly connected to the condenservia an inlet line and fluidly connected to the compressor via a suctionline. The evaporator is further in heat exchange relationship with afluid that is deliverable to a space. The compressor, condenser, inletline, evaporator, and suction line define a refrigeration circuit, andthe refrigeration system includes a refrigeration mode that is operableto circulate refrigerant through the refrigeration circuit to transferheat from the fluid to the refrigerant in the evaporator to cool thespace and to accumulate frost on the evaporator.

The refrigeration system further includes a bypass line having a firstend fluidly connected to the suction line between the evaporator and thecompressor, and a second end fluidly connected to the inlet line betweenthe condenser and the evaporator. A defrost circuit of the refrigerationsystem is defined by the evaporator, a portion of the suction linebetween the evaporator and the first end, the bypass line, and a portionof the inlet line between the second end and the evaporator. Therefrigeration system further includes a defrost mode that is operable tocirculate refrigerant through the defrost circuit to transfer heat fromthe refrigerant to the frost on the evaporator. The evaporator isconfigured to allow the refrigerant to flow by gravity from theevaporator to one of the portion of the suction line and the portion ofthe inlet line in the defrost mode.

In another embodiment, the refrigeration system further includes abypass line having a first end fluidly connected to the suction linebetween the evaporator and the compressor, and a second end fluidlyconnected to the inlet line between the condenser and the evaporator. Adefrost circuit of the refrigeration system is defined by theevaporator, a portion of the suction line between the evaporator and thefirst end, the bypass line, and a portion of the inlet line between thesecond end and the evaporator. A heat source is in heat transferrelationship with the defrost circuit such that the heat source isoperable to transfer heat to the refrigerant in the defrost circuit. Therefrigeration circuit further includes a defrost mode that is operableto circulate refrigerant through the defrost circuit to transfer heatfrom the refrigerant to the frost on the evaporator.

In yet another embodiment the invention provides a method of operating arefrigeration system. The method includes providing a defrost circuitincluding the evaporator, a portion of the suction line between theevaporator and the first end, the bypass line, and a portion of theinlet line between the second end and the evaporator, providing a heatsource in heat transfer relationship with the defrost circuit, andoperating the refrigeration system in a defrost mode. The method furtherincludes circulating refrigerant through the defrost circuit in thedefrost mode, transferring heat in the defrost mode from the heat sourceto the refrigerant in the defrost circuit, and transferring heat in thedefrost mode from the refrigerant to the frost on the evaporator.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a refrigeration system including arefrigeration circuit and a defrost circuit; and

FIG. 2 is an enlarged perspective view of the defrost circuit of FIG. 1.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

FIG. 1 shows a refrigeration system 10 that may be used in commercial,industrial, residential or any other applications providing a containeror case (not shown) having an open or closed configuration forrefrigeration of food product and other materials disposed in a space ofthe container or case. The refrigeration system 10 is adapted to be usedin any of a variety of configurations (e.g., refrigerated display case,refrigerated merchandiser freezer, cooler, temperature-controlledstorage, etc.) and includes a refrigerant in heat transfer relationshipwith a fluid to cool the space.

The refrigeration system 10 includes a refrigeration circuit 15 and adefrost circuit 20. The refrigeration circuit 15 is defined by acompressor 25, a condenser 30, an evaporator 35, a suction line 40, andan inlet line 45. The compressor includes a high pressure side in fluidcommunication with the condenser 30 and a low pressure or suction sidein fluid communication with the evaporator 35. The condenser 30 iscoupled to the compressor 25 and the evaporator 35, and includes aseries of looped conduits 50 to facilitate heat transfer between therefrigerant and the environment. The refrigerant is cooled by thecondenser 30 while maintaining the refrigerant at a relatively highpressure. In some constructions, the condenser 30 is located on arooftop to discharge the energy to the surrounding atmosphere.

The evaporator 35 is fluidly coupled with the compressor 25 via thesuction line 40 and is fluidly coupled with the condenser 30 via theinlet line 45, and includes coils 55 in heat exchange relationship withthe fluid. The suction line 40 couples an outlet of the evaporator 35with the low pressure side of the compressor 25 to deliver evaporatedrefrigerant from the evaporator 35 to the compressor 25. The inlet line45 couples with an exit side of the condenser 30 and an inlet of theevaporator 35 to convey condensed refrigerant from the condenser 30 tothe evaporator 35. The inlet line 45 is coupled with the evaporator 35at a position vertically above the attachment of the suction line 40 tothe evaporator 35 such that refrigerant flow through the evaporator 35is generally in a downward direction.

An expansion valve 60 is disposed in the inlet line 45 to create apressure differential and to control the pressure of the refrigerantdelivered to the evaporator 35. The expansion valve 60 shown in FIGS. 1and 2 includes a thermostatic expansion valve, although otherconfigurations of the expansion valve 60 are possible.

FIGS. 1 and 2 show the defrost circuit 20 that is defined by theevaporator 35, a bypass line 65 having a first end 70 and a second end75, a portion 80 of the suction line 40 between the evaporator 35 andthe first end 70, and a portion 85 of the inlet line 45 between thesecond end 75 and the evaporator 35. The first end 70 is in fluidcommunication with the suction line 40 between the compressor 25 and theevaporator 35. The second end 75 is in fluid communication with theinlet line 45 between the condenser 30 and the evaporator 35, and isfurther disposed between the expansion valve 60 and the evaporator 35.

The suction line portion 80 is attached to the evaporator 35 lower thanthe attachment of the inlet line portion 85 to the evaporator 35. Otherembodiments may include the suction line portion 80 attached to theevaporator 35 higher than the attachment of the inlet line portion 85 tothe evaporator 35.

The refrigeration system 10 further includes a suction valve 90, abypass valve 95, a first heater 100, a second heater 105, and acontroller 110. Other embodiments of the refrigeration system 10 mayinclude one or more valves (not shown) positioned on the inlet line 45upstream of the expansion valve 60. The suction valve 90 is disposed inthe suction line 40 downstream of the first end 70 and includes an openposition and a closed position to regulate refrigerant flow in therefrigeration system 10. In some embodiments, the suction valve 90 is acheck valve, although the suction valve may be a solenoid or other valve(e.g., hand valve) having an open position and a closed position.

The bypass valve 95 is disposed in the bypass line 65 adjacent the firstend 70 and includes an open position and a closed position to regulateflow of refrigerant in the refrigeration system 10. The bypass valve 95is similar to the suction valve 90 and may include a check valve, asolenoid valve, or other valve (e.g., hand valve) having open and closedpositions.

Other embodiments of the refrigeration system 10 may include a singlevalve similar to the suction valve 90 and the bypass valve 95 in therefrigeration system 10. In one construction, for example, the valve maybe disposed in the suction line 40 to regulate refrigerant flow withouta corresponding valve disposed in the bypass line 65. In otherconstructions, the singular valve may be disposed in the bypass line 65without a corresponding valve disposed in the suction line 40. In stillother constructions, more than one valve can be disposed in at least oneof the suction line 40 and the bypass line 45.

The first heater 100 is positioned adjacent the suction line portion 80such that the first heater 100 is at about the low point in the suctionline 40 and upstream of the suction valve 90. The first heater 100 isfurther in heat transfer relationship with the suction line portion 80.

The second heater 105 is positioned adjacent the bypass line 65 and isin heat transfer relationship with the bypass line 65 to superheat therefrigerant when the refrigeration system 10 is in the defrost mode.Other embodiments of the refrigeration system 10 may include one or moreheaters disposed on at least one of the bypass line 65, the suction lineportion 80, and the inlet line portion 85.

FIG. 1 illustrates the controller 110 that is in electricalcommunication with the compressor 25 to selectively operate thecompressor 25, and is in electrical communication with the suction valve90 and the bypass valve 95 to selectively vary the suction valve 90 andthe bypass valve 95 between the corresponding open positions and theclosed positions. The controller 110 is further in electricalcommunication with the first and second heaters 100, 105 to selectivelyactivate the first and second heaters 100, 105. In some embodiments, thecontroller 110 may be electrically coupled with other components of therefrigeration system, such as to the one or more valves positioned onthe inlet line 45.

In one embodiment, the controller 110 includes a timing device (notshown). The timing device defines a predetermined time interval or cycleto regulate operation of the refrigeration system 10. In otherembodiments, the controller 110 may include one or more sensors (notshown) to regulate operation of the refrigeration system 10.

The refrigeration system 10 operates in a refrigeration mode and in adefrost mode. The refrigeration mode circulates the refrigerant throughthe refrigeration circuit 15 and transfers heat from the fluid to therefrigerant. During operation in the refrigeration mode, the compressor25 is engaged by the controller 110 and compresses low-pressurerefrigerant drawn through the suction line 40. In some embodiments, anoperation time in the refrigeration mode is stored by the controller 110and compared to the predetermined time interval. When the operation timeexceeds the predetermined time interval, the controller 110 switches therefrigeration system 10 from the refrigeration mode to the defrost mode.

The suction valve 90 is open and the bypass valve 95 is closed in therefrigeration mode to circulate the refrigerant through therefrigeration circuit 15. The refrigerant is compressed by thecompressor 25 and flows through the condenser 30 where the refrigerantis condensed by the looped conduit 50. The condenser 30 releases energyfrom the refrigerant and condenses the superheated refrigerantdischarged from the compressor 25. The cooled refrigerant then passesthrough the inlet line 45 and the expansion valve 60. The expansionvalve 60 lowers the pressure of the refrigerant prior to the refrigerantreaching the evaporator 35. The expansion valve 60 varies therefrigerant from a high pressure refrigerant liquid to a low pressurerefrigerant two-phase liquid vapor prior to entry into the evaporator35.

The evaporator 35 transfers heat from the fluid passing over the coils55 to the refrigerant in the coils 55. The heat transfer from the fluidto the refrigerant in the coils 55 cools the fluid and causes watervapor to condense on the coils 55. Over time, the water vaporaccumulates as frost (not shown) on the evaporator 35. The suction line40 receives evaporated refrigerant from the evaporator 35 and deliversthe refrigerant to the compressor 25. The evaporated refrigerant flowsfrom the evaporator 35 by pressure from high pressure refrigerantentering the evaporator 35 from the inlet line 45. In embodiments thatinclude the inlet line 45 attached to the evaporator 35 higher than theattachment of the suction line 40, refrigerant in the refrigeration modeflows generally downward through the evaporator 35 and is assisted bygravity. In embodiments that include the inlet line 45 attached to theevaporator 35 lower than the attachment of the suction line 40,refrigerant in the refrigeration mode flows generally upward through theevaporator 35.

Buildup of frost during operation of the refrigeration system 10 in therefrigeration mode must be removed to allow efficient operation of theevaporator 35. The controller 110 switches the refrigeration system 10from the refrigeration mode to the defrost mode in response frostbuildup on the coils 55. In some embodiments, the timing devicegenerates a signal that is received by the controller 110 to vary therefrigeration system 10 between the refrigeration mode and the defrostmode. In other embodiments, the sensors generate a signal indicative ofthe amount of frost buildup. The controller 110 compares the signal witha predetermined level of frost buildup and selectively varies therefrigeration system 10 between the refrigeration mode and the defrostmode in response to the signal. In still other embodiments, the sensormay detect a predetermined temperature and/or pressure of the fluid thatcorresponds to frost buildup on the evaporator 35. The sensor thengenerates a signal indicative of the temperature and/or pressure that isreceived by the controller 110. The controller 110 varies therefrigeration system 10 between the refrigeration mode and the defrostmode in response to the signal.

The controller 110 disengages the compressor 25 when the defrost mode isactivated. The defrost mode circulates the refrigerant through thedefrost circuit 20 to transfer heat from the refrigerant to the frostaccumulated on the evaporator 35. The defrost mode directly transfersheat between the refrigerant and the frost without substantially heatingair adjacent the evaporator 35. The heat transfer from the refrigerantto the frost on the coils 55 melts the frost and at least partiallyliquefies the refrigerant. The controller 110 further restricts flow ofrefrigerant through the refrigeration circuit 15 by adjusting thesuction valve 90 from the open position to the closed position, andadjusting the bypass valve 95 from the closed position to the openposition. Closing the suction valve 90 and opening the bypass valve 95ensures that the refrigerant will flow through the defrost circuit 20and will not flow through the refrigeration circuit 15.

The defrost circuit 20 bypasses a substantial portion of therefrigeration circuit 15 using the bypass line 65. The suction valve 90closes to allow refrigerant to flow through the bypass line 65 and intothe evaporator 35 without passing through the compressor 25, thecondenser 30, and the expansion valve 60. In embodiments that includevalves positioned on the inlet line 45, the valves may be opened orclosed to regulate flow of refrigerant into the defrost circuit 20. Therefrigerant in the defrost circuit 20 circulates through the suctionline portion 80, the bypass line 65, the inlet line portion 85, and theevaporator 35 to melt the frost that accumulated on the coils 55 duringthe refrigeration mode. The refrigerant flows by gravity from theevaporator 35 toward a low point of the defrost circuit 20, circulatingthe refrigerant through the remaining portions of the defrost circuit20.

The refrigerant exiting the evaporator 35 at the end of therefrigeration mode is initially heated by the heat transfer between thefluid and the refrigerant. This refrigerant may initially havesufficient heat such that the refrigerant circulates through the defrostcircuit 20 by pressure differences to melt the frost on the coils 55without activation of the first and second heaters 100, 105. Theinitially heated refrigerant is defined by a relatively high pressurethat generates flow through the defrost circuit 20. In some embodiments,additional valves may be used to generate flow of refrigerant throughthe defrost circuit 20.

The heat transfer from the heated refrigerant to the frost on the coils55 at least partially liquefies or condenses the refrigerant withoutsubstantially affecting the temperature of the fluid passing over thecoils 55. In embodiments that include the suction line portion 80positioned lower than the inlet line portion 85, the at least partiallycondensed refrigerant in the defrost mode flows generally downwardthrough the evaporator 35 to the suction line portion 80 and is furtherassisted by gravity. In addition, heated refrigerant in the defrost modefrom the inlet line portion 85 forces condensed refrigerant into thesuction line portion 80 due to a pressure difference between the heatedrefrigerant entering the evaporator 35 in the defrost mode and thecooled refrigerant leaving the evaporator 35. The condensed refrigerantin the defrost mode flows through the evaporator 35 in a direction(i.e., refrigerant flow from the inlet line 85 to the suction line 80)that is the same as the direction of refrigerant flow during operationof the refrigeration system 10 in the refrigeration mode.

In embodiments that include the inlet line portion 85 positioned lowerthan the suction line portion 80, the at least partially condensedrefrigerant in the defrost mode flows generally downward through theevaporator 35 to the inlet line portion 85 and is further assisted bygravity. In addition, heated refrigerant in the defrost mode from thesuction line portion 80 forces condensed refrigerant into the inlet lineportion 85 due to a pressure difference between the heated refrigerantentering the evaporator 35 in the defrost mode and the cooledrefrigerant leaving the evaporator 35. The condensed refrigerant in thedefrost mode flows through the evaporator 35 in a direction (i.e.,refrigerant flow from the suction line portion 80 to the inlet lineportion 85) opposite the direction of refrigerant flow during operationof the refrigeration system 10 in the refrigeration mode.

The controller 110 selectively engages the first and second heaters 100,105 in the defrost mode to transfer heat to the condensed refrigerantpassing through the suction line portion 80 and the bypass line 65. Theheat transfer from the first and second heaters 100, 105 superheatsrefrigerant in the defrost circuit 20. The first heater 100 at leastpartially superheats refrigerant in the suction line portion 80 when therefrigeration system 10 is in the defrost mode. The second heater 105 atleast partially superheats refrigerant in the bypass line 65 when therefrigeration system 10 is in the defrost mode. The controller 110manages the temperature of the superheated refrigerant in the defrostcircuit 20 through selective activation of the first and second heaters100, 105. The superheated refrigerant then flows through the remainderof the bypass line 65 and into the evaporator 35. During operation ofthe refrigeration system 10 in the defrost mode, the refrigerantcirculates continuously through the defrost circuit 20 to defrost thecoils 55. Other embodiments may operate the refrigeration system 10 inthe defrost mode by circulating the refrigerant through the defrostcircuit 20 at predetermined intervals to maximize the defrost of thecoils 55.

The controller 110 provides a means of cycling one or more of thesuction valve 90, the bypass valve 95, the first and second heaters 100,105, and other components of the refrigeration system 10 (e.g., valvespositioned on the inlet line 45). Cycling one or more of thesecomponents achieves a desired effect regarding defrost of the coils 55in the defrost mode. The valves 90, 95 can be varied between the openand closed positions multiple times during a single defrost modeoperation. Similarly, the first and second heaters 100, 105 may beengaged and disengaged several times during one defrost mode operationto achieve a temperature of the refrigerant in the defrost circuit thatmaximizes defrost of the coils 55.

In some embodiments, the first and second heaters 100, 105 superheatrefrigerant in the defrost circuit 20 such that very little or no liquidrefrigerant remains. Without sufficient liquid refrigerant in thedefrost circuit 20, the temperature of the superheated refrigerantsubstantially increases above a saturation temperature of therefrigerant. As a result, circulation of refrigerant heated well abovethe saturation temperature stalls and the coils 55 cannot be effectivelydefrosted. In these embodiments, additional liquid refrigerant may beintroduced from the inlet line 45 to maintain the refrigerant in thedefrost circuit 20 at about the saturation temperature and to facilitaterefrigerant flow through the defrost circuit. Valves positioned on theinlet line 45 can be varied between open and closed positions to allowan appropriate amount of additional liquid refrigerant to enter thedefrost circuit 20. Alternatively, the bypass line 65 may besufficiently large to accommodate additional liquid refrigerant.

Once the coils 55 are sufficiently defrosted during the defrost mode(e.g., after the predetermined interval has elapsed), the controller 110opens the suction valve 90, closes the bypass valve 95, and disengagesthe first and second heaters 100, 105. The controller 110 furtherengages the compressor 25 and the remaining components of therefrigeration circuit 15 to switch from the defrost mode to therefrigeration mode. The refrigeration mode and defrost mode are repeatedas necessary to maintain the space at predetermined conditions and toremove buildup of frost on the coils 55.

Thus, the invention provides, among other things, a refrigeration systemthat includes a refrigeration circuit, an evaporator, and a defrostcircuit to defrost the evaporator. Various features and advantages ofthe invention are set forth in the following claims.

1. A refrigeration system comprising: a compressor; a condenser fluidlyconnected to the compressor; an evaporator fluidly connected to thecondenser via an inlet line and fluidly connected to the compressor viaa suction line, the evaporator in heat exchange relationship with afluid deliverable to a space, wherein the compressor, condenser, inletline, evaporator, and suction line define a refrigeration circuit,wherein the system includes a refrigeration mode operable to circulaterefrigerant through the refrigeration circuit to transfer heat from thefluid to the refrigerant in the evaporator to cool the space andaccumulate frost on the evaporator; and a bypass line having a first endfluidly connected to the suction line between the evaporator and thecompressor and a second end fluidly connected to the inlet line betweenthe condenser and the evaporator, wherein the evaporator, a portion ofthe suction line between the evaporator and the first end, the bypassline, and a portion of the inlet line between the second end and theevaporator define a defrost circuit, wherein the system includes adefrost mode operable to circulate refrigerant through the defrostcircuit to transfer heat from the refrigerant to the frost on theevaporator, and wherein the evaporator is configured to allow therefrigerant to flow by gravity from the evaporator to one of the portionof the suction line and the portion of the inlet line in the defrostmode.
 2. The refrigeration system of claim 1, further comprising a valvedisposed on the suction line wherein the valve is operable to open inthe refrigeration mode and to close in the defrost mode.
 3. Therefrigeration system of claim 1, further comprising a valve disposed onthe bypass line, wherein the valve is operable to close in therefrigeration mode and to open in the defrost mode.
 4. The refrigerationsystem of claim 1, further comprising a first valve in fluidcommunication with the suction line and a second valve in fluidcommunication with the bypass line, wherein the first valve is operableto open in the refrigeration mode and to close in the defrost mode, andthe second valve is operable to close in the refrigeration mode and toopen in the defrost mode.
 5. The refrigeration system of claim 1,further comprising a heat source in heat transfer relationship with thedefrost circuit, the heat source operable to transfer heat to therefrigerant in the defrost circuit.
 6. The refrigeration system of claim5, wherein the heat source is adjacent at least one of the portion ofthe suction line and the bypass line to superheat refrigerant in thedefrost mode.
 7. The refrigeration system of claim 6, wherein theevaporator is configured and oriented to allow the superheatedrefrigerant to at least partially displace refrigerant in theevaporator.
 8. The refrigeration system of claim 5, wherein the heatsource includes a first heat source adjacent the portion of the suctionline and a second heat source adjacent the bypass line to superheatrefrigerant in the defrost mode.
 9. A refrigeration system comprising: acompressor; a condenser fluidly connected to the compressor; anevaporator fluidly connected to the condenser via an inlet line andfluidly connected to the compressor via a suction line, the evaporatorin heat exchange relationship with a fluid deliverable to a space,wherein the compressor, condenser, inlet line, evaporator, and suctionline define a refrigeration circuit, wherein the system includes arefrigeration mode operable to circulate refrigerant through therefrigeration circuit to transfer heat from the fluid to the refrigerantin the evaporator to cool the space and accumulate frost on theevaporator; a bypass line having a first end fluidly connected to thesuction line between the evaporator and the compressor and a second endfluidly connected to the inlet line between the condenser and theevaporator, wherein the evaporator, a portion of the suction linebetween the evaporator and the first end, the bypass line, and a portionof the inlet line between the second end and the evaporator define adefrost circuit; and a heat source in heat transfer relationship withthe defrost circuit, the heat source operable to transfer heat to therefrigerant in the defrost circuit, wherein the system includes adefrost mode operable to circulate refrigerant through the defrostcircuit to transfer heat from the refrigerant to the frost on theevaporator.
 10. The refrigeration system of claim 9, wherein the heatsource is adjacent at least one of the portion of the suction line andthe bypass line to evaporate refrigerant in one of the portion of thesuction line and the bypass line.
 11. The refrigeration system of claim9, wherein the heat source includes a first heat source adjacent theportion of the suction line and a second heat source adjacent the bypassline.
 12. The refrigeration system of claim 9, further comprising avalve disposed on the suction line, wherein the valve is operable toopen in the refrigeration mode and to close in the defrost mode.
 13. Therefrigeration system of claim 9, further comprising a valve disposed onthe bypass line, wherein the valve is operable to close in therefrigeration mode and to open in the defrost mode.
 14. Therefrigeration system of claim 9, further comprising a first valve influid communication with the suction line and a second valve in fluidcommunication with the bypass line.
 15. The refrigeration system ofclaim 9, wherein the heat transfer from the refrigerant to the frost isoperable to liquefy the refrigerant, and wherein the evaporator isconfigured and oriented to allow the liquefied refrigerant to flow bygravity from the evaporator to one of the portion of the suction lineand the portion of the inlet line in the defrost mode.
 16. A method ofoperating a refrigeration system to cool a space, the refrigerationsystem including a compressor, a condenser fluidly connected to thecompressor, an evaporator fluidly connected to the condenser via aninlet line and fluidly connected to the compressor via a suction line,and a bypass line having a first end fluidly connected to the suctionline between the evaporator and the compressor and a second end fluidlyconnected to the inlet line between the condenser and the evaporator,the evaporator in heat transfer relationship with a fluid that is influid communication with the space, the method comprising: providing arefrigeration circuit including the compressor, condenser, inlet line,evaporator, and suction line; operating the refrigeration system in arefrigeration mode; circulating refrigerant through the refrigerationcircuit in the refrigeration mode; transferring heat in therefrigeration mode from the fluid to the refrigerant in the evaporator;cooling a space in the refrigeration mode; accumulating frost on theevaporator in the refrigeration mode; providing a defrost circuitincluding the evaporator, a portion of the suction line between theevaporator and the first end, the bypass line, and a portion of theinlet line between the second end and the evaporator, providing a heatsource in heat transfer relationship with the defrost circuit; operatingthe refrigeration system in a defrost mode; circulating refrigerantthrough the defrost circuit in the defrost mode; transferring heat inthe defrost mode from the heat source to the refrigerant in the defrostcircuit; and transferring heat in the defrost mode from the refrigerantto the frost on the evaporator.
 17. The method of claim 16, whereintransferring heat in the defrost mode from the refrigerant to the frostfurther includes melting the frost on the evaporator and condensing therefrigerant.
 18. The method of claim 16, further comprising providing avalve between the evaporator and the compressor; opening the valve inthe refrigeration mode; and closing the valve in the defrost mode. 19.The method of claim 16, further comprising providing a valve in thebypass line; opening the valve in the defrost mode; and closing thevalve in the refrigeration mode.
 20. The method of claim 16, furthercomprising providing a first valve between the suction line and thecompressor and providing a second valve in the bypass line.
 21. Themethod of claim 20, wherein operating the refrigeration system in therefrigeration mode further includes opening the first valve and closingthe second valve.
 22. The method of claim 20, wherein operating therefrigeration system in the defrost mode further includes closing thefirst valve and opening the second valve.
 23. The method of claim 16,wherein providing a heat source in heat transfer relationship with thedefrost circuit further includes providing a heat source in heattransfer relationship with the portion of the suction line.
 24. Themethod of claim 16, wherein providing a heat source in heat transferrelationship with the defrost circuit further includes providing a heatsource in heat transfer relationship with the bypass line.
 25. Themethod of claim 16, wherein transferring heat in the defrost mode fromthe heat source to the refrigerant further includes evaporating therefrigerant in the portion of the suction line.
 26. The method of claim16, wherein evaporating the refrigerant in the portion of the suctionline further includes displacing the refrigerant in the evaporator tothe suction line.